The present invention relates to an apparatus for detecting abnormality of an oxygen sensor which measures the oxygen concentration of exhaust gas discharged from an internal combustion engine and also for controlling the air/fuel ratio of air and fuel mixture supplied to the internal combustion engine according to data showing abnormality of the oxygen sensor.
The air/fuel ratio of an air and fuel mixture supplied to an internal combustion engine is generally controlled based on a signal sent from an oxygen sensor provided in the exhaust system of the engine so as to lower the emission of exhaust discharge of the engine. As shown in FIG. 19, the air/fuel ratio is controlled in accordance with the output signal of the oxygen sensor in order to maintain the air/fuel ratio near the stoichiometric ratio at which purification of exhaust components reaches the optimum stage.
When the oxygen sensor used for feed-back controlling the air/fuel ratio is abnormal, the emission of exhaust discharge may increase. Various techniques have hence been proposed for diagnosing abnormality of the oxygen sensor and furthermore for, when abnormality of the oxygen sensor is detected, compensating the feed-back control of the air/fuel ratio. Examples of apparatus for diagnosing abnormality of the oxygen sensor are illustrated in Japanese Published Unexamined Patent Applications No. Sho-62-151770 and No. Sho-53-95421; and apparatus for compensating the air/fuel ratio control are in Japanese Published Unexamined Patent Applications No. Sho-58-222939 and No. Sho-59-3137.
When the oxygen sensor is contaminated by various substances, the sensor output shifts to lean or rich as shown in FIG. 20; that is, the performance of the oxygen sensor varies. The feed-back control of the air/fuel ratio according to an output signal of the oxygen sensor is thereby not performed satisfactorily, and thus the emission of exhaust discharge increases.
For example, when the oxygen sensor contaminated by silicon is used for feed-back control of the air/fuel ratio, nitrogen oxides (NOx) in the exhaust discharge increase; and when the oxygen sensor contaminated by lead is used, carbon monoxide (CO) in the exhaust discharge increases.